Process for removing acids from lithium salt solutions

ABSTRACT

The present invention provides a process for generating acid-free lithium salt solutions for lithium and lithium ion batteries and for preparing high purity lithium salts. The invention comprises removing acid species from lithium salt solutions such as lithium hexafluorophosphate solutions using weak base resins. The process does not require the addition of a base such as ammonia which when added to the electrolytic solution generally must be removed from the final product. Once the lithium salt has been treated by the weak base resin, the substantially acid-free lithium salt solution may be recovered from the weak base resin to provide a solution which may be used as an electrolytic solution or which may be used to prepare high purity lithium salts. The highly pure salts and solutions have a hydrogen fluoride concentration of less than 10 ppm

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to commonly owned copending provisionalapplication Ser. No. 60/031,763 filed Nov. 26, 1996, and claims thebenefit of its earlier filing date under 35 U.S.C. § 119(e).

FIELD OF THE INVENTION

This invention relates to lithium and lithium ion batteries, andparticularly to a process for generating acid-free lithium saltsolutions for use in lithium and lithium ion batteries and for preparinghigh purity lithium salts.

BACKGROUND OF THE INVENTION

Solutions of lithium hexafluorophosphate (LiPF₆), lithiumhexafluoroarsenate (LiAsF₆), lithium hexafluoroantimonate (LiSbF₆) andlithium tetrafluoroborate (LiBF₄) dissolved in a variety of organicsolvents have found utility in primary and secondary lithium batteries.In particular, nonaqueous electrolytic solutions comprising lithiumhexafluorophosphate exhibit high electrochemical stability andconductivity. Nevertheless, the lithium hexafluorophosphate used inelectrolytic solutions is susceptible to both thermal decomposition andhydrolysis, each of which are catalyzed by the presence of acidicimpurities in the lithium salt or solution.

The thermal decomposition of LIPF₆ occurs at elevated temperaturesaccording to Equation 1. ##EQU1## The thermal decomposition of LiPF₆ istypically minimized by storing the lithium salt and the lithium saltsolutions under refrigerated or sub-ambient conditions.

The hydrolysis of lithium hexafluorophosphate occurs according toEquations 2-5. ##EQU2## Hydrolysis generally occurs because of thepresence of moisture and acidic impurities in the lithium salt orsolution. Therefore, it is preferred that the lithium salt solution befree of all acidic impurities in order to achieve high levels ofstability and performance. Nevertheless, hydrogen fluoride is a reactantin the formation of commercial grades of lithium hexafluorophosphate andtrace amounts of at least 100 ppm generally remain in the lithium salt.See, e.g., Introduction to Hashimoto, distributed by Biesterfeld U.S.,Inc. (1994).

As shown in Equations 2-5, once hydrolysis is initiated (Equation 2),the rate of hydrolysis progressively increases because hydrogenfluoride, which catalyzes the reaction, is formed as a by-product of thehydrolysis reaction. Furthermore, the Li-P-O intermediates (e.g. LiPO₂F₂, LiHPO₃ F, LiH₂ PO₄) are more easily hydrolyzed than LiPF₆ and thusfacilitate the accumulation of HF in the lithium salt solution.Therefore, the lithium salts and lithium salt solutions preferablyshould be free of water, hydrogen fluoride and the Li-P-O intermediatesto increase the stability and the performance of the lithium salts andlithium salt solutions. Additionally, because hydrogen fluoride andother acids are also highly detrimental to the function of the activecomponents of lithium and lithium ion batteries, these acidic speciesshould be removed.

One method of removing acidic impurities and thus preventing thedecomposition and hydrolyzing of lithium hexafluorophosphate is to treatthe salt and/or solution with a base and then maintain the salt and/orsolution under basic conditions. For example, U.S. Pat. No. 5,378,445 toSalmon et al. describes the use of a base such as ammonia to prevent theacid-catalyzed decomposition of lithium hexafluorophosphate.Nevertheless, the presence of ammonia in the electrolytic solution maybe detrimental to battery performance. Furthermore, the reactionproducts of ammonia, e.g., NH₄ F formed by the reaction of hydrogenfluoride and ammonia, may be detrimental to battery performance.Therefore, ammonia and its reaction products generally must be removedfrom the final product.

SUMMARY OF THE INVENTION

The present invention provides a process for removing acidic speciesfrom lithium salt solutions for lithium and lithium ion batteries andfor the recovery of high purity lithium salts. The process does notrequire the addition of a base such as ammonia which when added to thelithium salt solution generally must be removed from the final product.

The process for removing acidic species from a lithium salt solutioncomprises contacting a solution comprising a lithium salt, a solvent,and at least one acidic species, with a weak base resin to remove theacidic species from the lithium salt solution. Typically, the lithiumsalt is selected from LiPF₆, LiAsF₆, LiSbF₆ and LiBF₄, and preferably isLiPF₆. Once treated by the weak base resin, the substantially acid-freelithium salt solution may be easily recovered from the weak base resin.The weak base resin may then be reused after washing with a basicsolution to strip the acid species from the weak base resin.

According to another aspect of the invention, a lithium salt solution isprovided comprising at least one lithium salt as described above and asolvent wherein the amount of hydrogen fluoride present in the solutionis no more than about 10 ppm. Suitable solvents include acetonitrile,dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylenecarbonate, ester-based solvents, and mixtures thereof. The lithium saltsolution may be combined with a positive electrode and a negativeelectrode to form an electrochemical cell for lithium batteries.

These and other features and advantages of the present invention willbecome more readily apparent to those skilled in the art uponconsideration of the following detailed description which describes boththe preferred and alternative embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that according to the process of the presentinvention, a weak base resin can be used to remove acids from lithiumsalt solutions such as lithium hexafluorophosphate solutions. Thetreated solutions have considerably low acidic levels thereby reducingthe decomposition and hydrolysis of the lithium salt and increasing thestability of the lithium salt. Preferred embodiments of the inventionare described below for the treatment of LiPF₆ solutions but theinvention is not limited thereto and may be used with lithium salts ingeneral such as LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, and combinations thereof.

The process for removing acids from a lithium hexafluorophosphatesolution comprises contacting a lithium hexafluorophosphate solutioncontaining one or more acids with a weak base resin to remove the acidsfrom the lithium hexafluorophosphate solution. Generally, theelectrolytic solution comprises lithium hexafluorophosphate, at leastone acidic species (e.g., hydrogen fluoride, LiPO₂ F₂ or LiHPO₃ F) and asolvent. Suitable solvents include acetonitrile, dimethyl carbonate,diethyl carbonate, ethylene carbonate, propylene carbonate, ester-basedsolvents (e.g. methyl acetate and methyl formate), and mixtures thereof.For example, the solvent may be a mixture of dimethyl carbonate andethylene carbonate.

The weak base resin used to strip the acid species from the lithiumhexafluorophosphate solution may be any anionic weak base resin known inthe art such as type A, E, P and S weak base resins, and the like. TypeS weak base resins are based on chloromethylation of a styrene resinfollowed by reaction of a secondary amine such as dimethyl amine. Type Sresins typically possess strong base (quaternary amine) functionalityinitially with functionality decreasing upon cycling of the resin. TypeP weak base resins are based on phenol-formaldehyde polymers with weakbase functionality of the polyalkyleneamine type. Type E resins areproduced by the reaction of epichlorohydrin with a polyalkyleneamine.Type A weak base resins are based on polyacrylates with the aminefunctionality (i.e. a mono-, di- or poly- amine) bonded to the polymerstructure by an ester or amide linkage. Preferably, the weak base resinis a polyvinylpyridine resin according to the following formula:##STR1## Suitable polyvinylpyridine resins are available under theReillex™ name from Reilly Industries, Inc. Another suitable weak baseresin is a tertiary amine in a styrene/divinyl benzene matrix such asAmberlyte IRA 94 from Rohm & Haas Co. Generally, because the weak baseresin may contain large amounts of water, the weak base resin is driedbefore it is used in the acid removal process. For example, the weakbase resin may be dried under a vacuum at a temperature of about 100° C.In addition, water may be further removed from the weak base resin byrunning a solvent through the weak base resin. Typically this solvent isthe same as the solvent which is to be used in the lithiumhexafluorophosphate solution which is to be run through the column.

Upon contact with a lithium hexafluorophosphate solution containinghydrogen fluoride and other acidic species, the weak base resin reactswith the acidic species. Specifically, the polyvinylpyridine resinreacts with hydrogen fluoride according to Equation 6 to formpolyvinylpyridinium poly(hydrogen fluoride). ##STR2## The lithiumhexafluorophosphate solution is recovered from the process with reducedacid concentration. The polyvinylpyridinium poly(hydrogen fluoride)resin may then be stripped of the acid species by reacting the resinwith a base. For example, ammonia may be used in an acetonitrile solventwith the stripping step proceeding according to Equation 7. ##STR3## Theammonium fluoride may then be recovered in the acetonitrile solvent andthe polyvinylpyridine reused in the acid removal process.

Preferably, an ion exchange column is used to contact the lithiumhexafluorophosphate solution with the weak base resin but any othersuitable method may be employed. When an ion exchange column is used,the column is packed with the weak base resin. The lithiumhexafluorophosphate solution is then fed from the top of the column andpasses through the weak base resin by gravity. In the column, the acidspecies from the lithium hexafluorophosphate solution reacts with aportion of the weak base resin and is removed from the solution. Whenthe lithium hexafluorophosphate solution exits at the bottom of thecolumn, a substantial portion of the acids have been removed. Thelithium hexafluorophosphate solution may again be fed into the column orinto one or more additional columns in a multi-stage process until thedesired amount of acid is removed. The lithium hexafluorophosphatesolution recovered from the weak base resin preferably possesses lessthan about 10 ppm of hydrogen fluoride. In addition to HF, the acidintermediates formed from the acid-catalyzed decomposition of LiPF₆,namely, LiPO₂ F₂ and LiHPO₃ F, can be removed from the solution in thetreatment process.

The weak base resin may then be reused after washing with a basicsolution. In the ion exchange column, the solution is generally added tothe top of the column and runs through the weak base resin by gravity.The basic solution is then recovered at the bottom of the column.Generally, the basic solution or additional basic solutions are runthrough the column until the acid species are removed from the weak baseresin. For example, the weak base resin may be treated until the pH ofthe basic solution recovered from the solution is the same as the pH ofthe basic solution when it is fed to the column thus indicating theremoval of the acid species from the weak base resin. Desirably, thebase used to strip the acid species from the weak base resin is providedin the same solvent that is used for the lithium hexafluorophosphatesolution.

The solvent to be used for the electrolytic solution is preferablypretreated to remove water prior to combining the solvent with the LiPF₆electrolyte. The water is typically removed by passing the solventthrough a molecular sieve compound such as a sodium, lithium orpotassium zeolite. For example, the molecular sieve compound may beprovided in a packed column and the solvent to be used in theelectrolytic solution fed at the top of the column and passed throughthe molecular sieve compound in the column by gravity. The solvent thatis recovered at the bottom of the column has a lower water content thanthe solvent fed into the column. The pretreatment may continue in thesame column or in additional columns in a multi-stage process until thewater content in the solvent has been reduced to acceptable levels.

The lithium hexafluorophosphate may be treated prior to storage and use.This is typically accomplished by dissolving the LiPF₆ salt in asuitable solvent and treating the LiPF₆ solution to remove any acidswhich are present in the solution. The LiPF₆ may then be stored and usedin solution or the solvent may be removed to form a high purity LiPF₆salt which, in turn, may be stored or used. Preferably, acetonitrile isthe solvent used for the preparation of high purity lithium salts. As aresult of treatment, the LiPF₆ is less susceptible to thermaldecomposition and hydrolysis during storage and where applicable, duringthe solvent removal process. In addition to or instead of treating LiPF₆solutions prior to storage, LiPF₆ solutions may also be treated afterstorage and prior to use to remove any acid which has formed during thestorage of the LiPF₆ salt.

The lithium salt solution once treated may be used as an electrolyticsolution in an electrochemical cell. Typical electrochemical cellsconsist essentially of a positive electrode, a negative electrode, andan electrolytic solution. The positive electrode preferably comprisesLiMn₂ O₄ spinel particles, a conductive agent (e.g., graphite or carbonblack), and a binder material (e.g., polyvinylidene difluoride (PVDF)).The process of the invention has been particularly useful with LiMn₂ O₄because of the lack of stability of the LiMn₂ O₄ spinel in highly acidicmedia. The negative electrode can be lithium metal or alloys, or anymaterial capable of reversibly lithiating and delithiating at anelectrochemical potential relative to lithium metal between about 0.0 Vand 0.7 V. Examples are carbonaceous materials including carbonaceousmaterials containing hydrogen, boron, silicon and tin, and tin oxides ortin-silicon oxides. The resulting electrochemical cell may be used aloneor combined with other cells to form a lithium battery having greaterstorage stability than conventional lithium batteries.

The present invention will now be further illustrated by the followingnon-limiting examples.

Example I Single Pass Column Treatment of LiPF₆ Solution

Reillex® 425 high porosity beads (polyvinyl pyridine) were dried under avacuum at 100°C. to remove excess water. The polyvinylpyridine beadswere packed into a glass column and an acetonitrile solvent was passedthrough the column to remove any water remaining in thepolyvinylpyridine beads. Thirty pounds of LiPF₆ were dissolved in anacetonitrile solvent and the hydrogen fluoride content in the solutionwas measured at 180 ppm. The LiPF₆ solution was fed into the top of thecolumn and passed through the column by gravity. The LiPF₆ solutionrecovered at the bottom of the column possessed a hydrogen fluoridecontent of less than 10 ppm.

Example II Multipass Column Treatment of LiPF₆ Solution

Reillex® 425 beads were dried and packed in a column as described inExample I. An ethylene carbonate/dimethyl carbonate solvent was passedthrough the column to remove excess water. A 1M LiPF₆ solution in anethylene carbonate/dimethyl carbonate solvent was measured for hydrogenfluoride content. The initial hydrogen fluoride content was 3730 ppm.The LiPF₆ solution was fed at the top of the column and subsequentlyrecovered at the bottom of the column. After the first pass in thecolumn, the LiPF₆ solution possessed a hydrogen fluoride content of 89ppm. The LiPF₆ solution was again fed into the column and recovered. Thehydrogen fluoride content of the LiPF₆ solution after the second passwas less than 10 ppm.

Example III Batch Treatment of LiPF₆ Solution

Reillex® 425 high porosity beads were dried as described in Example Iand placed in a beaker. Acetonitrile was added to the beaker, retainedfor a short period of time to remove any remaining water in thepolyvinylpyridine beads, and removed. A 1M LiPF₆ solution inacetonitrile having a hydrogen fluoride content of 1300 ppm was added tothe beaker. The LiPF₆ solution was retained in the beaker for a periodof 24 hours. The recovered LiPF₆ solution possessed a hydrogen fluoridecontent of less than 10 ppm.

Example IV Batch Treatment of LiPF₆ Solution Using Alternative Weak BaseResin

Amberlyte RIA-94 weak base resin beads were dried as described inExample I and placed in a beaker. Acetonitrile was added to the beaker,retained for a short period of time to remove residual water, andremoved. A 1M solution of LiPF₆ in acetonitrile was measured forhydrogen fluoride content and had an initial hydrogen fluoride contentof 1400 ppm. The solution was added to the beaker and retained for 24hours. The LiPF₆ solution was removed and the hydrogen fluoride contentmeasured at less than 10 ppm.

Example V LiPO₂ F₂ and LiPO₃ F Removal

Reillex® 425 high porosity beads were dried, packed in a column andexcess water extracted using acetonitrile as described in Example I.Thirty pounds of a 1M LiPF₆ solution in acetonitrile were measured forLiPO₂ F₂ and LiHPO₃ F content. The initial LiPO₂ F₂ content was 684 ppmand the initial LiHPO₃ F content was 2000 ppm. The LiPF₆ solution wasfed at the top of the column and recovered at the bottom of the column.After the first pass, the LiPO₂ F₂ content was less than 10 ppm and theLiHPO₃ F content was 680 ppm. The solution was again fed to the column.After the second pass, the LiPO₂ F₂ content was again less than 10 ppmand the LiHPO₃ F content was 140 ppm.

It is understood that upon reading the above description of the presentinvention, one skilled in the art could make changes and variationstherefrom. These changes and variations are included in the spirit andscope of the following appended claims.

That which is claimed:
 1. A solution consisting essentially of afluoride containing lithium salt and a solvent wherein the amount ofhydrogen fluoride present in the solution is no more than about 10 ppm.2. The solution according to claim 1 wherein the lithium salt isselected from LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, and mixtures thereof.
 3. Thesolution according to claim 1 wherein said lithium salt is LiPF₆.
 4. Thesolution according to claim 1 wherein the solvent is selected fromacetonitrile, dimethyl carbonate, diethyl carbonate, ethylene carbonate,propylene carbonate and mixtures thereof.
 5. A lithium salt selectedfrom LiPF₆, LiAsF₆, LiSbF₆ and LiBF₄ wherein the amount of HF present inthe lithium salt is no more than about 10 ppm.